Phosphate in the sediments of the Gulf of Lions (NW Mediterranean Sea), relationship with input by the river Rhone

Abstract

We investigated P-input by the Rhone river into the Mediterranean Sea taking into account P trapped in the surface sediment of the Gulf of Lions. Total phosphate concentration was determined every cm in the upper 10 cm-layer of sediments sampled at 11 stations in the Gulf of Lions during two cruises (March 1998 and January 1999). Two low downward gradients, one East–West and another North–South, with distance to the Rhone river mouth were found. Except at one station, total phosphate concentration in surface sediments was found to be constant with depth down to 10 cm. Values for individual stations ranged between 400 and 700 μg g−1 with an average value of 547 μg g−1 (st. dev. = 63 μg g−1) for the whole gulf. The low variability in total-P concentration in sediments is in contrast to the large variability in suspended matter load of the river Rhone and suggests the dominance of authigenic P removal mechanisms in P burial. The total P-pool in the upper 10 cm-layer of the sediments in the gulf was estimated at 562 kt, with about 80% trapped into the shelf and 20% into the slope. Annual P-deposition was estimated as 7.2–12.4 kt y−1, from the P-pool in the sediment and the sedimentation rates. This is equivalent to a previous estimation of the river Rhone input, estimated to be about 6.5–12.2 kt y−1. As the Rhone is the major river flowing into the Mediterranean Sea, total P in surface sediments of the Gulf of Lions should be taken into account in P-budgets at the scale of the Mediterranean Sea.